Counterfeiting of consumer goods, paper currencies, financial documents and identification cards is countered by a large variety of optical security measures designed to deter and defeat this illicit activity. Optical variable devices (OVDs) that change their apparent color when viewed from different angles are particularly effective optical security devices for anti-counterfeiting and brand protection applications. Among them, cholesteric liquid crystal polymer (CLCP) films or pigments have become more prevalent in the security printing industry. CLCP laminates or pigments offer two visual security levels based on a unique color shift effect (OVD) and on a selective polarized reflection effect. Both effects can be utilized as distinctive signatures for optical authentication. Said two effects cannot be reproduced by counterfeiters employing standard reproduction techniques or using non-CLCP materials. A third, forensic, security level is based on the CLCP unique and tunable reflection spectrum.
Users of optical security devices prefer labels with additional functionalities. In particular, labels having printed information or identifying patterns (e.g. barcodes or logos) that can be easily read or identified by consumers, are preferred over blank colored CLCP labels that are currently in use. Therefore, CLCP labels that can be fabricated by conventional printing techniques to provide informative, multicolor information, in addition to OVD and polarized reflection effects, are very useful.
While CLCP films or pigments are inherently colored, they are usually applied as monochrome images. Formulating CLC monomer materials into inks of different colors and using standard printing techniques to create multicolor images is difficult and expensive since these materials are usually solids at ambient temperature and are soluble only in non-aqueous solvents. Attempting to create multicolor CLCP images requires typically multiple printing runs of each color, multiple masks and precise mask registration. There is, therefore, a need for a practical and economical solution which can provide multicolor images based on CLCP materials.
It is desirable to achieve a multicolor CLCP images using conventional roll-to-roll printing equipment for high throughput and mass production of low cost security labels. The images of CLCP labels can consist of a fixed repetitive images, or they can be serialized images, where the image or information therein varies from one label to the next. The ability to serialize the label information is an important useful additional functionality.
It is well known that many cholesteryl derivatives organic compounds, that have a cholesteric liquid crystal (CLC) phase, exhibit a thermochromic effect whereby their reflection band, or color, shifts with temperature. An early report of such effect was published by Fergason et al. Increasing the temperature of most of these materials cause a shift of their reflection color to shorter wavelength (“blue-shift”). U.S. Pat. No. 4,140,016 lists many such material formulations used in the current ubiquitous colorful temperature indicators. The thermochromic effect is highly non-linear, where most of the color change takes place near the phase transition temperature from a smectic phase at lower temperatures to a CLC phase. The nonlinear feature makes it difficult to generate a uniform and predetermined color in the presence of small temperature variations.
The above mentioned thermochromic effect was extended also to CLCP materials. U.S. Pat. No. 6,117,920 discloses a method of creating a multicolor image in a single CLCP layer using the thermochromic effect. In a multi-mask, batch process, the un-polymerized CLC layer is heated to a temperature corresponding to a first color. The CLC layer is then exposed to polymerizing UV radiation through a mask that permits exposure only of the image parts designated with the first color. The process is repeated as many times as the number of distinct colors in the image. This process is clearly slow, expensive and of low yield since the highly nonlinear thermochromic effect requires exquisite temperature uniformity to generate a required color and control the uniformity of that color throughout the CLC layer. In a second implementation, a single grayscale mask is used where each grayscale level corresponds to a different color. The CLC layer is heated to a temperature corresponding to a first color and then is exposed to a precise UV dose that is sufficient for polymerizing only the parts of the image that correspond to the most transparent grayscale level. This process is repeated, at different temperature for each color, with higher UV doses for each subsequent color. The second implementation reduces the number of required masks at the cost of requiring collimated UV light source and a precise control of the exposure, thus leading to a slow, expensive and a low yield process. In addition, it is difficult to achieve with any of the above implementations a material system that provides a full color gamut from the red to green to blue.
Publication WO 00/34808 describes a method of generating multiple colors by applying UV radiation, at a constant temperature, to modify the cholesteric pitch in selected areas and hence also their colors. The change in color results from the reduced chirality of at least one of the chiral dopants, in the CLC mixture, after being irradiated. The chiral component is either been irreversibly broken into non-chiral products or is been reversibly transformed into an isomer of a lesser chirality. Only after forming a complete colored image, a different wavelength UV radiation is applied to polymerize the entire, now patterned, CLC layer and freeze the image. Color patterning is obtained by using a grayscale UV mask. In order to keep the cholesteric pitch and color constant across the CLC layer, the UV absorption of the image forming UV radiation must be very low, leading to a poor utilization of the UV energy. This method puts sever requirements on the materials used to assure that the pitch modification step does not cause polymerization. This can be done by forming the image either at low temperature where polymerization is inhibited or with a UV wavelength that has negligible polymerization effect or in an oxygen rich atmosphere that inhibits polymerization. These requirements are difficult and expensive to implement on a roll-to-roll production line and, in the end, not entirely effective. An additional problem is that the above process disturbs the quality of the planar cholesteric configuration leading to a lower color purity either due to a reflection band broadening or due to excessive light scattering from disclination lines in the converted cholesteric. The latter problem has been alleviated by annealing the CLC at a temperature below its phase transition before polymerization as disclosed in U.S. Pat. No. 6,836,307. However, it remains a slow process that requires very intense UV irradiation and is expensive to implement.
A similar approach to generating multicolor in a single CLCP layer is disclosed in U.S. Pat. No. 9,279,084. A trigger dopant molecule is used to change selectively the CLC pitch, and hence its color, by exposing it to a first wavelength of non-polymerizing UVA radiation. When the writing of a multicolor image is completed, an exposure to a second wavelength of polymerizing UVB radiation freezes the image. The trigger molecule is chiral molecule capable of inducing a twist in a non-chiral LC and thus inducing a CLC structure. When added to an already CLC phase of some color, depending on its sense of chirality, the trigger dopant can either further tighten the cholesteric twist or partially unwind it, thus shifting the base reflection color to the blue or to the red. Its ability to act so is quantified by an intrinsic property called ‘twisting power’. The unique feature of the trigger molecules disclosed in U.S. Pat. No. 9,279,084 is that they have two conformations (isomers) of different twisting powers, one value of twisting power in its ground state and a different one in its excited state. A transition between the two conformations in one direction is induced by exposure to UVA radiation. The opposite transition happens spontaneously with a decay time that is sensitive to temperature. The effective twisting power of the trigger dopant depends on the numbers of the trigger molecule populations in each of the two conformations. These numbers depend on the intensity of the UVA pulse and is clearly a dynamic quantity. The trigger molecule approach requires very high UV intensities and fast image formation to minimize the effect of relaxation of the excited state conformation and to limit the effect of molecular diffusion that can cause a drift of the colors from their intended values or blurred the border lines between two differently colored areas. In addition, this method has necessarily poor utilization of the UVA radiation. The absorption of the first wavelength by the CLC layer must be low to assure a uniform UVA intensity throughout the layer thickness. Otherwise, a non-uniform UVA intensity will induce a gradient in the pitch of the cholesteric layer that will lead to a broad reflection band of poor color purity and of low reflectivity. While U.S. Pat. No. 9,279,084 discloses also a process of generating multicolor images using conventional masks and UV sources, the dynamic nature of this method and the necessarily poor UVA absorption make it practical only if a high intensity UV laser and a laser scanning methods are used to write images. This approach of generating multicolor images requires expensive specialty chemicals such as the trigger dopant, narrowband photoinitiators in the UVB and puts sever requirements on all reactive components to inhibit polymerization under the high intensity addressing UVA radiation. Capital investment and maintenance cost of such a complicated production system are high since the image forming step is non-standard for roll-to roll production lines.
The current invention solves the problems in the prior art. The invention disclosed herein teaches a set of novel techniques for manufacturing of CLCP based labels where the visible information is printed in multiple colors on an opaque or transparent substrates. Multicolor images are created from a single, monochrome uniform pitch CLCP layer; and yet no special liquid crystal materials or dopants or special UV light source are required. The invention disclosed herein is particularly adaptable to continuous web, roll-to-roll, printing lines. As a result, the current invention makes possible high throughput production line, high quality images and a low cost of the final CLCP label.